Protecting metal during acid contact

ABSTRACT

PROTECTING METAL DURING CONTACT WITH ACID BY INCORPORATING AN INHIBITOR MIXTURE OF A POLYAMINOMETHYLENEPHOSPHONATE AND AN N,N&#39;&#39;-DICYCLOALKYL-THIOURA. THE INHIBITOR MIXTURE IS USED IN DESCALING, ACID CLEANING, ACID PICKLING, ETC.

nited States Patent Oflice 3,778,377 Patented Dec. 11, 1973 3,778,377 PROTECTING METAL DURING ACID CONTACT Herbert D. Clark, Burbank, Calif., assignor to Universal Oil Products Company, Des Plaines, Ill. No Drawing. Filed Oct. 6, 1971, Ser. No. 187,120 Int. Cl. C02b 5/06 US. Cl. 252-82 7 Claims ABSTRACT OF THE DISCLOSURE Protecting metal during contact with acid by incorporating an inhibitor mixture of a polyaminomethylenephosphonate and an N,N-dicycloalkyl-thiourea. The inhibitor mixture is used in descaling, acid cleaning, acid pickling, etc.

BACKGROUND OF THE INVENTION The present invention provides a novel inhibitor composition which is added to the acid solution and serves to protect the metal. A particular application is the removal of scale deposits in boilers used to generate steam, in cooling water systems or in other operations in which scale deposits are formed by the polyvalent ions contained in the water. The deposits generally are salts such as calcium sulfate, calcium carbonate, barium salts, sodium salts, etc. As another advantage to the present invention, scale formation is inhibited by acid treatment effected in accordance with the present invention. Scale deposits also area problem in oil producing equipment. Still other applications relate to acid cleaning, acid pickling, etc.

In one embodiment the present invention relates to a mixture of a polyaminomethylenephosphate and an N,N'- dicycloalkyl-thiourea.

In a preferred embodiment the polyaminomethylenephosphonate is in a major concentration and may comprise from about 55% to about 95% by weight and the N,N'-dicycloalkyl-thiourea may be in a concentration of from about 5% by weight to about 45% 'by weight. However, in another embodiment, these concentrations may be within the range of from about 50% to about 99% of the former and from about 1% to about 50% by weight of the latter. These percentages are on an active ingredient basis and exclusive of other components contained in the composition.

In another embodiment the present invention relates to thew method of acid cleaning metal surfaces which comprises contacting said metal surfaces with an acid solution containing the novel composition hereinbefore set forth.

In a specific embodiment the present invention relates to a method of removing scale in boilers used to generate steam from water, which comprises treating the same with an aqueous acid solution containing the inhibitor composition hereinbefore set forth.

While the polyaminomethylenephosphonate has been proposed heretofore for use in inhibiting precipitation and scale formation, as described in US. Pat. 3,336,221, itnow has been found that surprisingly improved results are obtained when the polyaminomethylenephosphonate is used in admixture with the N,N-dicycloalkyl-thiourea. As will be shown by the appended examples, a synergistic effect is obtained. This is a surprising result because, while the dicycloalkyl-thiourea has been proposed heretofore for use as an acid inhibitor, it has not been proposed for use to remove scale. Thus, the synergistic effect obtained when using these components in admixture would not be expected.

As hereinbefore set forth, one component of the novel composition of the present invention is a polyaminometh'ylenephosphonate. Numerous polyaminomethylenephosphonates are described in U.S. Pat. 3,336,221 and are incorporated herein by reference. The polyaminomethylenephosphonate may be illustrated by the following structure:

where m is 1 to 3 and n is 3 to 15. Also included are the ammonium or alkaline metal salts thereof, or mixtures of these. In a preferred embodiment it in the above structure is from about 8 to about 12. Typical properties of a preferred polyaminomethylene phosphonate, in which m in the above structure is 2 and n is about 10 is stated to be substantially as follows.

The polyaminomethylenephosphonate will be utilized as such or prepared as a solution or dispersion in a suitable solvent, emulsifier, dispersant or the like, which may be selected from those hereinafter set forth.

Another component of the inhibitor composition of the present invention is N,N-dicycloalkyl-thiourea and particularly N,N'-dicyclohexyl-thiourea. Other dicycloalkyl derivatives contain 3 to 5 or 7 to 12 carbon atoms in the cycloalkyl ring. The N,N-dicycloalkyl-thiourea preferably is utilized as a 5 to 25% by weight solution or suspension in a water soluble solvent, emulsifier, dispersant or the like.

Any suitable water soluble solvent, emulsifier, dispersant or the like may be used and may be selected from one or more of the following: (I) alcohols including methanol, ethanol, propanol, butanol, etc., (2) glycols including ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, dimethylethylene glycol, trimethyldiethylene glycol and higher molecular weight polyethylene glycols or polypropylene glycols, (4) ketones including acetone, methylethyl ketone, diethyl ketone, methylpropyl ketone, ethylpropyl ketone, dipropyl ketone, etc., (5) polyoxyethylene ether, polyoxypropylene ether, methoxy polyethylene glycol, etc., (6) dimethylformamide, dimethylacetamide, diethylacetamide, dipropylacetamide, dimethylpropionamide, diethylpropionamide, etc., or (7) mixtures thereof. A particularly preferred mixture used with the dicyclohexyl-thiourea is from about 5% to about 20% dicyclohexyl-thiourea, from about 40% to about by weight of N,N-dimethylacetamide and from about 5% to about 30% by weight of polyoxyethylene and/or polyoxypropylene derivative. The polyethylene glycols preferably have a molecular weight of from about 150 to about 1000 and more particularly from about 180 to about 800. The particular compound or compounds to be used will be selected with reference to the particular mixture of polyaminomethylenephosphonate and dicycloalkylthiourea and the particular acid employed, as well as the particular type of metal surface to be cleaned. The phosphonate and/or thiourea on an active ingredient basis may be in a concentration of from about 5% to about 95% by Weight and preferably from about 8% to about 60% by weight.

In still another embodiment the inhibitor composition may contain a color indicator to permit visual observation of loss of potency of the inhibitor composition. For example, Methyl Violet may be included in the composition to indicate loss of potency when the color changes from violet to redish. The amount of Methyl Violet will be sufficient for the purpose and may be used in an amount of from about 0.5 to about 10% by weight of the inhibitor composition, based on active ingredients, although lower or higher concentrations may be used as desired. It is understood that other suitable color indicators may be employed.

In a preferred method, the inhibitor mixture, with or without color indicator, is first prepared and then added to an aqueous solution of the acid. The specific amount of inhibitor composition to be included in the aqueous solution will be selected with reference to the particular use thereof. For example, when used for descaling, the inhibitor composition will be determined with regard to the amount and type of scale to be removed. The concentration of inhibitor composition may be within the range of from about 0.1 to about 1000 and preferably from about 0.1 to about 100 p.p.m. (parts per million) based on the acid, although lower or higher concentrations may be used as desired. The appended examples illustrate suitable solutions.

As hereinbefore set forth, in one application the inhibitor composition of the present invention is used in acid solutions to remove scale formation in boilers. The scale is formed from water having high concentrations of polyvalent ions, and may comprise one or more of calcium salts, barium salts, magnesium salts, sodium salts, etc., such as the carbonate, sulfate, oxalate, silicate, etc. The scale deposits lead to plugging, pumping difliculties, loss of heat transfer, etc. Similar problems occur in cooling water systems and the removal of the scale is necessary to insure eflicient operation. In another application, acid solutions are used to clean metal surfaces including, for example, in acid pickling systems. Regardless of the particular acid treatment, it is important to protect the metal from excessive loss of metal or other impairment thereof.

The acid treatment is effected in conventional manner using any suitable acid, including hydrochloric, sulfuric, phosphoric, etc. These acids treatments are well known in the industry and accordingly need not be described in detail herein. The inhibitor composition is incorporated in the acid solution in any suitable manner.

The following examples are introduced to illustrate further the novelty and utility of the present invention but not with the intention of unduly limiting the same.

EXAMPLE I A series of evaluations was made in the following manner. Zinc strips of equal dimensions and of approximate equal weight of 2 grams were used as the test strips. The acid solution comprised 200 ml. of a 10% distilled water dilution of Baum commercial muriatic acid. The zinc strips were immersed in the solution for a period of 3 hours at a temperature of 100 F.

The inhibitor components, as indicated below, were mixed with 467.5 grams of 15 Baum muriatic acid and the mixture diluted to 10% with distilled water.

The zinc strip immersed in the acid solution which contained no inhibitor was completely destroyed, as shown in the following table.

1 Po1yarninomethylenephosphonate of the structure hereinbefore set forth in which m is 2 and n is about 10.

2 N N -d.icyclohexyl-thiourea.

The dicyclohexyl-thiourea used in this example is a commercial product available under the trade name of Main 338 and is believed to comprise about 10% by weight of N,N-dicyclo-hexyl-thiourea, about 60% by weight of N,N-dimethylacetamide and about 30% by weight of a polyoxyethylene or polyoxypropylene derivative of a molecular weight of about 800. The amount of the DCT in Table 2 is the mixture of thiourea, dimethylacetamide and the polyoxyalkylene derivative.

From the data in the above table, it will be seen that the zinc strip immersed in the acid soution, not containing inhibitor, was completely destroyed. On the other hand, it will be noted that the PMP and DCT each separately served to protect the metal to varying degrees. However, it will be seen that the equal weight mixture considerably reduced the weight loss as shown in Test No. 4. A considerable reduction in weight loss was ob tained when using the mixture of Test No. 5. When compared to the use of the PMP alone, the mixture used in Test No. 6 resulted in considerable improvement. However, for protecting zinc it is preferred that the PMP and DCT be used in substantially equal amounts for optimum results.

EXAMPLE II Another series of evaluations were made in substantially the same manner as in Example I, but using mild steel strips. The steel strips were of equal dimensions and of approximate equal weight of 11 grams. Each strip was immersed in 200 ml. of a 20% distilled water dilution of 15% Baum commercial muriatic acid for a period of 15 hours at a temperature of F. The additives in the amounts shown in the following table were mixed with 467.5 grams of 15 Baum muriatic acid and the mixture diluted to 20% with distilled water. The results of these tests are reported in the following table.

TAB LE 3 Weight Test No. loss Inhibitor Here again a synergistic effect was obtained when using the mixture of PMP and DCT as compared to the use of either component alone. While these results are not as startling as compared to the results reported in Example I, a considerable improvement was obtained, ranging from 13% to 83% when compared with the use of PMP or DCT alone.

EXAMPLE III 5. from about 1% to about 50% by weight of N,N-dicyc1oalkyl-thiourea.

2.11The mixture of claim 1 in which said thiourea is N, N-dicyclohexyl-thiourea.

3. The mixture of claim 1 in which said polyaminomethyleuephosphonate is in a concentration of at least 55%.

4. The mixture of claim 2 in which said N,N'-dicyclohexyl-thiourea is prepared as a composition of from about 5% to about 20% by weight of N,N'-dicyclohexylthiourea, from about 40% to about 80% by weight of N,N-dimethylacetamide, and from about 5% to about 30% by weight of polyoxyethylene or polyoxypropylene derivative.

5. The method of removing scale from a metal surface in a boiler used to generate steam from water, which comprises treating said surface with an acid solution containing the synergistic mixture of claim 2.

6. The method of cleaning a metal surface by treatment with an acid solution, which comprises adding to the acid solution the synergistic mixture of claim 2.

7. In the acid pickling of metal, the improvement which comprises adding to the acid solution the synergistic mixture of claim 2.

References Cited UNITED STATES PATENTS 15 GEORGE F. LESMES, Primary Examiner W. R. DIXON, 1a., Assistant Examiner US. Cl. X.R. 

